This invention relates generally to sync signal recovery systems and specifically to systems for recovering synchronizing signals that have been distorted.
The characteristics of synchronizing signals in NTSC television signals are controlled by FCC standards and are applicable to both over-the-air and cable transmissions. The width or duration of the synchronizing signal tip is specified to be 4.7.+-.0.1 microseconds. In the 525 scanning line system used in the United States, the length of a horizontal line, that is the period between synchronizing signal tips, is 63.5 microseconds.
It is common in both over-the-air and cable transmissions, to encode or scramble television signals to prevent their unauthorized use by those not having the requisite decoding equipment. A very common form of scrambling involves suppression of sync signals so that conventional television receiver synchronizing signal circuits are incapable of recovering them. In those instances, a form of pilot signal or other sync signal reference is also transmitted to enable the decoding equipment to appropriately reconstitute the synchronizing signals. It is however, often desirable to have the capability of regenerating proper sync signal information from the video information alone, without the need for supplemental pilot or reference signals.
In the encoding system mentioned, the regenerated sync signals need to be identified as to vertical field to properly and automatically control the decoding equipment. This is because of field interlacing in the television signal which requires the regenerated sync signal to be correlated with the video display to result in a properly decoded signal.